Orthogonal Frequency Division Multiplexing (OFDM), the underlying technique behind OFDMA, is a multicarrier communication system which converts a high data rate stream into a group of parallel low data rate streams which transform the frequency selective fading situation being encountered in a wireless channel to tractable flat fading scenario. OFDMA has gained acceptance recently as an important multiple access technique to provide OFDM services to a group of subscribers simultaneously. The downlink of an OFDMA communication system is nearly equivalent to that of an OFDM system and it is characterized by a single carrier frequency offset (CFO) which can be easily estimated using conventional estimation techniques available for OFDM. The CFO estimation problem in the uplink transmission of OFDMA is more challenging than the downlink. In the uplink transmission of OFDMA each active subscriber station (SS) may have a different CFO relative to the common base station (BS) receiver which, if uncompensated, would result in self and multiple access interferences and thereby poor bit error rate (BER) performances.
The first step of OFDMA synchronization takes place in the downlink transmission where the subscriber stations SSs or mobile terminals (MTs) estimate the timing and frequency offsets using the pilot signals transmitted by the base station (BS). A major portion of the frequency offset can be estimated in this step and the CFO estimation techniques available for OFDM can be directly employed for this purpose as the user's signal appear at each SS with a common frequency error (T. M. Schmidl and D. C. Cox, “Robust frequency and timing synchronization for OFDM” IEEE Trans. Commun., vol. 45, no. 12, pp. 1613-1621, December 1997; M. Morelli and U. Mengali, “An improved frequency offset estimator for OFDM applications,” IEEE Commun. Left., vol. 3 no. 3, pp. 75-77, March 1999; X. Ma, C. Tepedelenlioglu, G. B. Giannakis, and S. Barbarossa, “Non-data-aided carrier offset estimators for OFDM with null subcarriers: Identifiability, Algorithms, and Performance,” IEEE J. Sel. Areas Commun., vol. 19, no. 12, pp. 2504-2511, December 2001; Defeng (David) Huang and K. B. Letaief, “Carrier frequency offset estimation for OFDM systems using null subcarriers,” IEEE Trans. Commun., vol. 54, no. 5, pp. 813-822, May 2006.). The SSs use these estimates as a reference for the uplink transmission. But due to Doppler shifts and oscillator phase noise, the signals received at the BS from various SSs will have small frequency offsets within the range of +/−0.5 of subcarrier spacing. Hence the second step of OFDMA CFO estimation is done at the BS by estimating the frequency offsets between the various SSs and the BS receiver. This turns out to be a challenging multiple CFO estimation problem.
CFO estimation in the uplink OFDMA is considered as an active research topic recently and a few methods are proposed in the literature. Most of these schemes are too specific to the carrier assignment schemes (CAS) used within the OFDMA system.
The main difference between OFDM and OFDMA is that in the case of later, in the downlink, each OFDM symbol conveys distinct sets of data to multiple subscribers simultaneously. Thus the available N subcarriers are usually evenly divided among the M users, by dividing N into R subchannels, each consisting of Q=N/R subcarriers. As the maximum number of users that an OFDMA system can support is decided by R, it is assumed that M≦R. Since the carriers allocated to the M users are to be distinct, Ik∩Ij=Ø for k≠j, where Ik and Ij are the sets of sub carriers allocated to kth and jth users respectively. During the uplink transmission M users transmit independently, using the sub-carriers allocated to them by padding zeroes in rest of the sub-carrier locations.
The three CAS proposed for OFDMA are subband CAS, generalized CAS and interleaved CAS.
A timing and frequency offset estimation scheme for OFDMA with subband CAS is proposed in (J. J. van de Beek, P. O. BAorjesson, M. L. Boucheret, D. LandstrAom, J. M. Arenas, O. A Odling, C. A Ostberg, M. Wahlqvist, and S. K. Wilson, “A time and frequency synchronization scheme for multiuser OFDM,” IEEE J. Sel. Areas Commun., vol. 17, no. 11, pp. 1900-1914, November 1999.) Here the subbands of each user are filtered first and estimation is done by using the cyclic prefix (CP) associated with each user's data. But the estimator performance deteriorates when the number of subcarriers in a subband becomes smaller due to the high correlation of samples. A CFO estimation scheme for the quasi-synchronous (where the CP duration is greater than two way propagation delay and channel delay spread) subband CAS OFDMA is proposed in (S. Barbarossa, M. Pompili, and G. B. Giannakis, “Channel-independent synchronization of orthogonal frequency division multiple access systems,” IEEE J. Sel. Areas Commun., vol. 20, no. 2, pp. 474-486, February 2002) This technique uses the virtual subcarriers employed in each subband and estimates the CFO by means of the energy minimization at the DFT output as done in (X. Ma, C. Tepedelenlioglu, G. B. Giannakis, and S. Barbarossa, “Non-data-aided carrier offset estimators for OFDM with null subcarriers: Identifiability, Algorithms, and Performance,” IEEE J. Sel. Areas Commun., vol. 19, no. 12, pp. 2504-2511, December 2001.)
A CFO estimation scheme for the quasi-synchronous interleaved OFDMA, which exploits the inherent periodic structure of this type of OFDMA symbol, is proposed in (Z. Cao, U. Tureli, and Y. D. Yao, “Deterministic multiuser carrier-frequency offset estimation for interleaved OFDMA uplink,” IEEE Trans. Commun., vol. 52, no. 9, pp. 1585-1594, September 2004). The accuracy of the method decreases when the number of users approaches the number of available subcarriers.
A maximum likelihood (ML) technique for the timing and frequency offset estimation in OFDMA system with a generalized CAS is proposed in (M. Morelli, “Timing and frequency synchronization for the uplink of an OFDMA system,” IEEE Trans. Commun, vol. 52, no. 2, pp. 296-306, February 2004). But it has a serious limitation as it assumes that all other users in the system are already synchronized perfectly. Modified forms of this method for quasi-synchronous scenarios are proposed in (M. O. Pun, M. Morelli, and C-C. J. Kuo, “Maximum-likelihood synchronization and channel estimation for OFDMA uplink transmissions,” IEEE Trans. Commun. Vol. 54, no. 4, pp. 726-736, April 2006; M. O. Pun, M. Morelli, and C-C. J. Kuo, “Iterative detection and frequency synchronization for OFDMA uplink transmissions,” IEEE Trans. Signal Proc., vol. 6, no. 2, pp. 629-639, February 2007), where timing and frequency offsets are estimated from the training blocks being transmitted by each user at the start of the uplink frame. This results in considerable wastage of bandwidth. Moreover, the computational complexity of (M. O. Pun et al, 2006) is very high. An overview of OFDMA and its synchronization techniques are presented in (M. Morelli, C-C. J. Kuo, and M. O. Pun, “Synchronization Techniques for Orthoganal Frequency Division Multiple Access (OFDMA): A Tutorial Review,” Proc. IEEE, vol. 95, no. 7, pp. 1394-1427, July 2007).
A pictorial representation of the three CAS schemes are shown in FIG. 3 for N=16 and Q=R=4. Here each color represents the subcarriers allocated to a specific user and each small rectangle represents one subcarrier.
All the CFO estimation schemes proposed for OFDMA, are carrier assignment scheme (CAS) specific. This introduces considerable in convenience in system design and bandwidth assignment.
In view of the growing popularity, efficient techniques for the CFO estimation with excellent performances are still needed for practical OFDMA system implementations. The present invention proposed a null sub carrier based CFO estimation technique for OFDMA which can be applied to subband CAS, generalized CAS and a modified form of interleaved CAS. Being a null sub carrier based estimator, the proposed CFO estimation scheme does not require the knowledge of fading channel coefficients for deriving the frequency offset estimates. A null sub carrier based CFO estimator where the estimation is done by minimizing a cost function which compute the total energy present in null sub carrier locations due to the CFO. The computational complexity and training overhead of the present invention method are much lower as compared to some of the CFO estimation schemes available in the literature for OFDMA transmissions.